The B51 means power brakes ONLY, (read power front drum brakes). If you had front disc brakes, the code was B41. And yes, you could have power brakes with front drum brakes. Bottom line, if the only code you have on your fender tag is B51, the car was born with front drum brakes, but with power assist. It was possible to order a car with no power brakes (with front drum brakes only), but if you had front disc brakes, power brakes came with it. Hope that nelps.
Robert